Maybe at some time you have visited a zoo. There you saw enormous elephants, tall giraffes, lions, monkeys, some types of birds and other kinds of animals. The animals in the different cages were so different that you must surely believe that they can have little in common.

So far, all living things in fact have something in common. All living things are made of tiny units, called cells. The huge elephant is made of hundreds of billions of cells, and there are little animals whose whole body is but a single cell. The human body, too, is made of billions of cells.

What do cells look like?

Most cells are so small that you need a potent magnifying lens (microscope) to see one. Other cells, however, are large enough to be seen with the unaided eye. Among these large cells are the root hairs of plants, certain seaweed, and the eggs of animals.

Cells are of many shapes. Some are round, some look like bricks with rounded corners, others are long and hair like and some others are shaped like plates, cylinder, ribbons or spiral rods.